[clang] [analyzer][NFC] Introduce APSIntPtr, a safe wrapper of APSInt (1/4) (PR #120435)

[Balazs Benics via cfe-commits]

steakhal wrote:

> I see that `unsafeConstructor` is used in the follow-up PRs. I'd move the addition of that function to the first PR that is using it.

I think I'd prefer keeping it here because this API would be here all at once.
Think of dependencies: All the rest of the commits use these APIs. If I'd move this `unsafeConstructor` to the next commit, then all the dependents would be dependent on that commit too, not just this initial commit.
All the rest are only dependent on this first commit so far, potentially ease later reverts (of those dependent commits).

https://github.com/llvm/llvm-project/pull/120435